ILLUMINATION. 



[ 339 ] 



INFLAMMATION. 



ILLUMINATION. So much has been 

 said in the Introduction (p. xxv), and in the 

 articles ANGULAR APERTURE, DIATO- 

 MACE.E, POLARIZATION and TEST-OB- 

 JECTS, upon the subject of illumination in 

 general, and of the effects of various kinds 

 of illumination in rendering evident the dif- 

 ferent structural peculiarities of objects, that 

 we shall merely add here two illustrations of 

 the importance of attention to the physiolo- 

 gical effects of a variation in the relative 

 amount of light transmitted by or reflected 

 from the parts of an object, or of contrast 

 as it is popularly called. 



If two candles, the flames of which are 

 of equal size and height, be placed parallel 

 to and 2 or 3 feet from a painted wall 

 or other un coloured surface, and a piece of 

 string be suspended at an inch or two from 

 the wall, and opposite the interspace between 

 the two candles, on lighting one of the latter, 

 the surface of the wall will be illuminated 

 at all points except those corresponding to 

 the shadow of the string. But on lighting 

 the other candle, two shadows will become 

 visible, the second lying in the direction of a 

 line drawn from the second candle through 

 the string to the wall. Thus by throwing an 

 increased amount of light upon all parts of 

 the wall except the line corresponding to the 

 shadow of the second candle, this line will 

 appear dark or even black; whereas it might 

 have been expected that this portion would 

 have appeared to the eye as light as it was 

 before, which is not the case. Again, if we 

 take a section of the shell of a hen's egg or 

 any similar object, and illuminate it by re- 

 flected light, all the more opake parts will 

 appear white and luminous ; but on trans- 

 mitting light from the mirror through the 

 object, the reflected light being unchanged, 

 the whole appearance will be altered, the 

 parts which were before white will now 

 become black, and vice versa. 



These experiments show that it cannot be 

 concluded from the dark appearance of parts 

 of an object that light is not reflected from or 

 transmitted through them ; for parts may be 

 made to appear dark or black by simply 

 throwing more light upon the surrounding 

 parts, so that darkness may indicate either 

 absolute or comparative absence of light. 

 This important point must always be borne 

 in mind in determining the cause of the 

 appearances of objects under different kinds 

 of illumination. 



INDIAN RUBBER, or CAOUTCHOUC. 

 This substance occurs naturally in globules 



suspended in the milky juices of many plants, 

 especially of the Orders Euphorbiacese, Ur- 

 ticaceae and Apocynaceae. The form of the 

 globules is varied. In PI. 39. fig. 23 is 

 represented part of a milk-vessel of Euphor- 

 bia antiquorum with two caoutchouc globules. 

 When such milky juices are evaporated, the 

 globules become blended into a uniform 

 elastic mass, the India-rubber. 



Solution of caoutchouc is sometimes used 

 as a cement for closing glass cells, but its 

 chief importance in this respect depends on 

 its forming a constituent of marine glue (see 

 CEMENTS). 



INDIGO. This well-known vegetable 

 substance is chiefly obtained from plants of the 

 genera Indigofera and Isatis, and Polygonum 

 tinctorium, but may be found in many others. 

 It has also been found in human urine, of 

 which it is probably a normal constituent. 

 Its best marked character is that of subliming 

 in flattened prisms and plates (PL 6. fig. 14). 

 Indigo is sometimes used as a colouring 

 matter for injections ; and is also very 

 useful for colouring the internal cavities of 

 Infusoria which swallow the granules; for 

 rendering visible ciliary motion (see INTRO- 

 DUCTION, p. xxxi); &c. The simplest mode 

 of employing it is to rub it from a water- 

 colour cake of indigo very gently with a 

 little water. The Infusoria require to be 

 left in the coloured mixture some time, and 

 it is well to remove them into clean water 

 for examination. 



BIBL. See CHEMISTRY. 

 INFLAMMATION. The phenomena of 

 inflammation are best studied in one of the 

 lower animals, as in the web of the frog's 

 foot, the mesentery of the frog, the tail of 

 the tadpole, or of the larva of the water- 

 newt (Triton), the process being excited by 

 the application of a hot needle, a solution of 

 common salt, ammonia, dilute spirit or vola- 

 tile oil. 



The first thing noticed is a contraction of 

 the capillaries, amounting to about one-third 

 of their calibre, the blood flowing through 

 them with increased velocity. The capillaries 

 then become dilated, redder and more full of 

 blood, but the current is still uniform. In 

 the next stage, the flow of blood becomes 

 irregular and oscillatory ; that is, the blood 

 goes forwards and backwards, sometimes 

 stopping for a time and then resuming its 

 onward course. At last the circulation 

 entirely ceases, the capillaries appearing 

 fully distended, and frequently tortuous and 

 varicose. When the stasis is perfect, the 



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